Non-invasive analysis, which for purposes of this application includes non-destructive analysis, is a valuable technique for acquiring information about systems or targets without undesirable side effects, such as damaging the system being analyzed. Non-invasive analysis has a broad range of applications including, non-destructive analysis of artifacts for defects, verification of the authenticity of documents, such as, bank notes, bio-metric analysis and bio-medical analysis of living entities. In the case of analyzing living entities, such as human tissue, and in particular the human eye, undesirable side effects of invasive analysis include the risk of infection along with pain and discomfort associated with the invasive process.
It is frequently useful to make biometric measurements on the human eye, such as lens curvature, lens thickness and axial length. Such measurements are useful, for example, in choosing an appropriate artificial inter-ocular lens to replace an existing natural lens that has been degraded by cataracts. It is also valuable to make routing visual evaluation of aspects of the eye, such as the retina.
A conventional approach is to use ultrasound or an Optical Coherence Tomography (OCT) based system to measure some parameters, such as axial length and a conventional camera to assist in alignment. However, ultrasound measurements require application of a liquid to the eye for index matching and are therefore somewhat invasive, OCT systems can be costly and require depth alignment with two surfaces either simultaneously or at high speed, and a conventional camera requires focusing.
Furthermore, where images and measurements are to be taken without the aid of a skilled operator, complex focusing and alignment procedures may not be practical. For example, home or mobile use of a monitor by an untrained subject (who may have less than perfect vision) would preclude any elaborate focusing and alignment procedures.
Digital Light Field Photography is well known. Recent developments in computer science and fabrication techniques of detector and micro-lens arrays make Digital Light Field Photography technical feasible for consumer devices. A good treatment of Digital Light Field Photography may be found in the dissertation of Ren Ng, entitled “Digital Light Field Photography”, copyright 2006 submitted to Stanford University in partial fulfillment of the requirements for the degree of doctor of philosophy.
Digital Light Field Photography does not require a skilled operator for focusing. Moreover Digital Light Field Photography provides information regarding the distances between images that can be used to facilitate alignment. However using a plenoptic camera (i.e field of light imaging device) to capture images of the eye is complicated by the fact that the eye is itself an optical instrument that includes a lens. In particular, the existence of the lens complicates using a field of light imaging device to image the retinal of an eye.
There remains, therefore, an unmet need for a low cost imaging or analysis system suitable for non-invasive sub-surface imaging or measurement that does not require focusing and that addresses complicating optical issues of an eye. but that.